Conductive absorption layer for flexible displays

ABSTRACT

The present invention relates to a bistable reflective display and a method for making the display comprising a substrate, an electrically modulated imaging layer, and a coatable color contrast conductive layer comprising an electronically conductive polymer and colorant.

FIELD OF THE INVENTION

The present invention relates to a combined conductive/color contrastlayer for use in reflective bistable electrically modulated displays.

BACKGROUND OF THE INVENTION

Currently, information is displayed using assembled sheets of papercarrying permanent inks or displayed on electronically modulatedsurfaces such as cathode ray displays or liquid crystal displays. Othersheet materials can carry magnetically written areas to carry ticketingor financial information, however magnetically written data is notvisible.

Media systems exist that maintain electronically changeable data withoutpower. Such system can be electrophoretic (E-ink), Gyricon or polymerdispersed cholesteric materials. An example of electronically updateabledisplay can be found in U.S. Pat. No. 3,600,060, that shows a devicehaving a coated then dried emulsion of cholesteric liquid crystals inaqueous gelatin to form a field responsive, bistable display. U.S. Pat.No. 3,816,786 discloses a layer of encapsulated cholesteric liquidcrystal responsive to an electric field. The electrodes can betransparent or nontransparent and formed of various metals or graphite.It is disclosed that one electrode must be light absorbing and it issuggested that the light absorbing electrode be prepared from paintscontaining conductive material such as carbon.

Fabrication of flexible, electronically written display sheets isdisclosed in U.S. Pat. No. 4,435,047. A substrate supports a firstconductive electrode, one or more layers of encapsulated liquidcrystals, and a second electrode of electrically conductive ink. Theconductive inks form a background for absorbing light, so that thedisplay areas appear dark in contrast to non-display areas. Electricalpotential applied to opposing conductive areas operates on the liquidcrystal material to expose display areas. Because the liquid crystalmaterial is nematic liquid crystal, the display ceases to present animage when de-energized. Dyes in either the polymer encapsulant orliquid crystal material absorb incident light. The dyes are part of asolution, and not solid submicron particles. U.S. Pat. No. 4,435,047further discloses the use of a chiral dopant in example 2. The dopantimproves the response time of the nematic liquid crystal, but does notcreate a light reflective state. The display structures disclosed arenot bistable in the absence of an electrical field.

U.S. Pat. No. 5,251,048 discloses a light modulating cell having apolymer dispersed chiral nematic liquid crystal. The chiral nematicliquid crystal has the property of being electrically driven between aplanar state reflecting a specific visible wavelength of light and alight scattering focal conic state. Chiral nematic liquid crystals, alsoknown as cholesteric liquid crystals, have the capacity of maintainingone of multiple given states in the absence of an electric field. Blackpaint is applied to the outer surface of rear substrate to provide alight absorbing layer outside of the area defined by the intersection ofsegment lines and scanning lines.

U.S. Pat. No. 6,753,937 to Grupp discloses a reflective liquid crystaldisplay devices, an absorbent black layer which is usually deposited onthe lower face of the back substrate, arranged at a higher level thanthe level of the back substrate. In this way, the number of so calledparasite reflections or back scatter of the incident light at theinterfaces between two materials or mediums of different indices isreduced. This allows the display contrast to be improved. Gruppdiscloses a polymer dispersed liquid crystal device having a first groupof transparent electrodes, a second group of electrodes, and a blackcolored absorbent layer made of electrically nonconductive material thatis an insulating lacquer, Heatsinkpaste® HSP 2741 by Lack Verke GmbH.The invention of Grupp requires separate processes for depositing theabsorbent black layer and the second conductor. The absorbent blacklayer is suggested by Grupp to be coated by silk-screen printing, tampoprinting, flexographic printing or vapor deposition. In addition, thereis no mention of providing an index match between the absorbent blacklayer and the polymer used in the polymer dispersed liquid crystallayer.

U.S. Pat. No. 6,788,362 discloses a thin, dark light absorbing layerbetween two thinly spaced, parallel electrodes operating on polymerdispersed cholesteric liquid crystal displays, in which, if the lightabsorbing layer for a display having polymer dispersed cholestericliquid crystals is not field carrying and not electrically conductive,it is possible to position such layer between electrodes to provideimproved image quality. Accordingly, the light absorbing layer does notcarry a field beyond limits defined by the intersection or overlap ofthe two electrodes. The disclosed display has polymer dispersed liquidcrystals, comprising a transparent substrate, a polymer dispersedcholesteric liquid crystal disposed over the substrate and definingfirst and second surfaces, a transparent conductor disposed over thefirst surface of the state changing layer, a second conductor on thesecond surface of the state changing layer, and a nonconductive,non-field spreading layer comprising a submicron pigment and binderdisposed between the polymer dispersed cholesteric liquid crystal layerand the second conductor to provide a light absorbing layer. Fine,preferably submicron, particles of pigment in a binder provide anelectro-chemically stable light absorber that maximizes light absorptionin the pigment-containing layer, while preventing field spreading beyondthe perimeter of the second electrode.

In typical conventional displays, the absorptive black layer is providedon the support from a viscous coating composition by a suitable printingtechnique. The conductive electrode, which is typically ITO, isseparately applied onto the support under vacuum.

PROBLEM TO BE SOLVED

There remains a need for a combined color contrast layer, also referredto as a black absorptive layer, and conductive layer of a display devicewith the functionality of both that affords ease of manufacture.

SUMMARY OF THE INVENTION

The present invention relates to a bistable reflective displaycomprising a substrate, a transparent conductive layer, an electricallymodulated imaging layer, and a coatable color contrast conductive layercomprising an electronically conductive polymer and colorant. Thepresent invention also relates to a method for making a bistablereflective display comprising providing a substrate, applying atransparent conductive layer applying an electrically modulated imaginglayer to the transparent conductive layer, and adding a color contrastconductive layer comprising an electronically conductive polymer andcolorant.

ADVANTAGEOUS EFFECT OF THE INVENTION

The present invention includes several advantages, not all of which areincorporated in a single embodiment. The instant invention combines thetwo previously separate layers (i.e., a conductive layer and a colorcontrast layer) into one, thus affording manufacturing advantage.Additionally, the coating composition is aqueous in nature, and can becoated at high speed on wide webs utilizing traditional manufacturingfacilities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate a single color display device with twodifferent viewing sides.

FIG. 3 illustrates a multicolored display device.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an aqueous coatable layer comprising lightabsorbing colored pigments and an electronically conductive polymer. Thelayer is designed to function as (1) a light absorption layer, alsoreferred to herein as a color contrast layer, typically associated witha bistable reflective display and (2) a conductive electrode. Thesurface electrical resistivity (SER) and transmission of the layer canbe widely tuned by changing the ratio of pigment, preferably carbonblack, to the conductive polymer. Typically, the light absorptive layeris positioned on the side opposite the side from which the display willbe viewed, that is, the viewing side of the display.

The color contrast conductive layer of the present invention has atleast dual functionality: it functions as a conductive layer providingsufficient electrical conductivity to carry a field across the lightmodulating layer; it also functions as a color contrast layer byproviding enough light absorption to generate necessary contrast.

The conductivity of the layer is provided by electronically conductivepolymers. Suitable electronically conductive polymers that are preferredfor incorporation in the layer of the invention are those withconjugated backbones, such as those disclosed in U.S. Pat. Nos.6,025,119, 6,060,229, 6,077,655, 6,096,491, 6,124,083, 6,162,596,6,187,522, and 6,190,846, incorporated herein in by reference. Theseelectronically conductive polymers include substituted or unsubstitutedaniline-containing polymers as disclosed in U.S. Pat. Nos. 5,716,550,5,093,439 and 4,070,189), substituted or unsubstitutedthiophene-containing polymers as disclosed in U.S. Pat. Nos. 5,300,575,5,312,681, 5,354,613, 5,370,981, 5,372,924, 5,391,472, 5,403,467,5,443,944, 5,575,898, 4,987,042 and 4,731,408, incorporated herein in byreference, substituted or unsubstituted pyrrole-containing polymers asdisclosed in U.S. Pat. Nos. 5,665,498 and 5,674,654, incorporated hereinin by reference, and poly(isothianaphthene) or derivatives thereof.These conducting polymers may be soluble or dispersible in organicsolvents or water or mixtures thereof. Preferred conducting polymers forthe present invention include pyrrole-containing polymers,aniline-containing polymers and thiophene-containing polymers. Morepreferred in the list is electronically conductive polythiophene,preferably polythiophene present in a cationic form with a polyanion.Typically, these polymers are dispersible in aqueous medium because ofthe presence of the polyanion, and hence, are environmentally desirable.

A preferred electronically conductive polythiophene is prepared by theoxidative polymerization of 3,4-dialkoxythiophene or3,4-alkylenedioxythiophene in the presence of a polyanion. The mostpreferred electronically conductive polymers include poly(3,4-ethylenedioxythiophene styrene sulfonate) which comprises poly(3,4-ethylenedioxythiophene) in a cationic form with polystyrenesulfonic acid. Theadvantage of choosing the polymer arises from the fact that it isprimarily water based, stable polymer structure to light and heat,stable dispersion and cause minimum concern for storage, health,environmental and handling.

Preparation of the polythiophene based polymers has been discussed indetail in a publication titled “Poly(3,4-ethylenedioxythiophene) and itsderivatives: past, present and future” by L. B. Groenendaal, F. Jonas,D. Freitag, H. Pielartzik and J. R. Reynolds in Advanced Materials,(2000), 12, No. 7, pp. 481-494, and references therein.

In a preferred embodiment, the electronically conductive polymercomprises:

a) a polythiophene according to Formula I

in a cationic form, wherein each of R1 and R2 independently representshydrogen or a C1-4 alkyl group or together represent an optionallysubstituted C1-4 alkylene group or a cycloalkylene group, preferably anethylene group, an optionally alkyl-substituted methylene group, anoptionally C1-12 alkyl- or phenyl-substituted 1,2-ethylene group, a1,3-propylene group or a 1,2-cyclohexylene group; and n is 3 to 1000;

and

b) a polyanion compound;

Polyanions used with these electronically conductive polymers includethe anions of polymeric carboxylic acids such as polyacrylic acids,poly(methacrylic acid), and poly(maleic acid), and polymeric sulfonicacids such as polystyrenesulfonic acids and polyvinylsulfonic acids, thepolymeric sulfonic acids being preferred for use in this inventionbecause of its stability and availability in large scale. Thesepolysulfonic acids may also be copolymers formed from vinylsulfonic acidmonomers copolymerized with other polymerizable monomers such as theesters of acrylic acid and styrene. The molecular weight of thepolyacids providing the polyanions preferably is 1,000 to 2,000,000 andmore preferably 2,000 to 500,000. The polyacids or their alkali saltsare commonly available, for example as polystyrenesulfonic acids andpolyacrylic acids, or they may be produced using known methods. Insteadof the free acids required for the formation of the electricallyconducting polymers and polyanions, mixtures of alkali salts ofpolyacids and appropriate amounts of monoacids may also be used. Thepolythiophene to polyanion weight ratio can widely vary between 1:99 to99:1, however, optimum properties such as high electrical conductivityand dispersion stability and coatability are obtained between 85:15 and15:85, and more preferably between 50:50 and 15:85. The most preferredelectronically conductive polymers include poly(3,4-ethylenedioxythiophene styrene sulfonate) which comprises poly(3,4-ethylenedioxythiophene) in a cationic form and polystyrenesulfonic acid.

Desirable results such as enhanced conductivity of the polythiophenelayer can be accomplished by incorporating a conductivity enhancingagent (CEA). Preferred conductivity enhancing agent s are organiccompounds containing dihydroxy, poly-hydroxy, carboxyl, amide, or lactamgroups, such as

(1) those represented by the following Formula II:(OH)_(n)—R—(COX)_(m)  II

wherein m and n are independently an integer of from 1 to 20, R is analkylene group having 2 to 20 carbon atoms, an arylene group having 6 to14 carbon atoms in the arylene chain, a pyran group, or a furan group,and X is —OH or —NYZ, wherein Y and Z are independently hydrogen or analkyl group; or

(2) a sugar, sugar derivative, polyalkylene glycol, or glycerolcompound; or

(3) those selected from the group consisting of N-methylpyrrolidone,pyrrolidone, caprolactam, N-methyl caprolactam, dimethyl sulfoxide orN-octylpyrrolidone; or

(4) a combination of the above.

Particularly preferred conductivity enhancing agents are: sugar andsugar derivatives such as sucrose, glucose, fructose, lactose; sugaralcohols such as sorbitol, mannitol; furan derivatives such as2-furancarboxylic acid, 3-furancarboxylic acid and alcohols. Ethyleneglycol, glycerol, di- or triethylene glycol are most preferred becausethey provide the maximum conductivity enhancement.

The conductivity enhancing agent can be incorporated by any suitablemethod. Preferably the conductivity enhancing agent is added to thecoating composition comprising the polythiophene. Alternatively, thecoated polythiophene containing layer can be exposed to the conductivityenhancing agent by any suitable method, such as post-coating wash.

The concentration of the conductivity enhancing agent in the coatingcomposition may vary widely depending on the particular organic compoundused and the conductivity requirements. However, convenientconcentrations that may be effectively employed in the practice of thepresent invention are about 0.5 to about 25 weight %; more conveniently0.5 to 10 and more desirably 0.5 to 5 as it is the minimum effectiveamount.

Even though the electronically conductive polymer of the invention mayhave some color associated with itself, the electronically conductivepolymer is not considered as a colorant for the purpose of thedescription of the present invention.

In addition to the electronically conductive polymer the color contrastconductive layer of the invention further comprises light absorbingpigment colorants. Making a neutral black using pigment colorants can beachieved by blending of colors, for example red, green and blueabsorbing pigment colorants. These pigment colorants can be chosen forhigh extinction coefficient and to minimize the absorption in the UV andIR spectral regions. The spectral purity of pigment colorants is howevernever perfect and mixtures often leave regions, which may be referred toas spectral holes, of weaker absorption between the primary absorptionmaximums in the visible spectral region. This reduces the overallabsorption and can lead to color shifts away from neutral underdifferent lighting conditions. These problems can be minimized by addingadditional pigment colorants with peak absorption wavelengthscorresponding to the spectral holes; however, designing specialtycolorant pigment systems that are compatible with coating or otherapplication technologies can be difficult and expensive. Alternatively,panchromatic carbon pigments can be used alone or in combination withother colorants in accordance with the present invention.

In one embodiment, the present invention combines colorants havingdiscrete absorption bands in the visible spectrum with carbon, having apanchromatic absorption throughout the near UV, visible and near IRspectral regions. Organic and inorganic colorants can be blended to makea wide variety of colors including black. They can be chosen to havelittle unwanted UV and IR absorptions, however, colorant, typicallypigments, tend to have spectral peaks making it difficult to formulate amixture that absorbs panchromatically using a limited set of pigmentcolors. U.S. Ser. No. 10/851,566, incorporated herein in by referencediscloses a useful color contrast layer, particularly a black layer,made by mixing colorants such as nonconductive colorants, preferablypigments, and carbon, where the carbon component is limited to no morethan 25% by weight. Such compositions can be effectively used in thecolor contrast conductive layer of the present invention.

The present invention utilizes a color contrast layer, frequentlyreferred to in the art as a dark, visible-light absorbing layer or ananopigmented layer, which has been rendered conductive through theincorporation of an electronically conductive polymer. Color contrastlayers may be radiation reflective layers or radiation absorbing layers.In some cases, the rearmost substrate of each display, that is, the sidemost opposite either the substrate or the viewer, is preferably paintedblack. The black layer absorbs infrared radiation that reaches the backof the display. In the case of the stacked cell display, the contrastmay be improved by coloring the back substrate of the last visible cellblack. The dark layer can be rendered transparent to infrared radiationfor some applications. This effectively provides the visible cell with ablack background that improves its contrast, and yet, does not alter theviewing characteristics of the infrared display. Materials such as blackpaint, which is transparent in the infrared region, is known to thoseskilled in the art. For example, many types of black paint used to printthe letters on computer keys are transparent to infrared radiation.

The color contrast conductive layer of the invention can be incorporatedin a device that displays one or more color. Examples of single colorand multicolor display devices comprising electrically imageable layersare discussed in U.S. Ser. Nos. 10/851,566 and 10/954,722, bothincorporated herein in by reference, and references therein andincorporated herein by reference. The color contrast conductive layer ofthe present invention can be incorporated in any of the displaysdiscussed in these references.

Suitable pigments used in the color contrast conductive layer may be anycolored materials, which are practically insoluble in the medium inwhich they are incorporated. The preferred pigments are organic in whichcarbon is bonded to hydrogen atoms and at least one other element suchas nitrogen, oxygen and/or transition metals. The hue of the organicpigment is primarily defined by the presence of one or morechromophores, a system of conjugated double bonds in the molecule, whichis responsible for the absorption of visible light. Suitable pigmentsinclude those described in Industrial Organic Pigments: Production,Properties, Applications by W. Herbst and K. Hunger, 1993, WileyPublishers. These include, but are not limited to, Azo Pigments such asmonoazo yellow and orange, diazo, naphthol, naphthol reds, azo lakes,benzimidazolone, diazo condensation, metal complex, isoindolinone andisoindolinic, polycyclic pigments such as phthalocyanine, quinacridone,perylene, perinone, diketopyrrolo-pyrrole, and thioindigo, andanthriquinone pigments such as anthrapyrimidine, triarylcarbonium andquinophthalone. The color contrast conductive layer preferably providesa substantially neutral hue, that is, a background which issubstantially neutral to the human eye. This can be accomplished bycombining at least two colorants, which have different hues, in thelayer.

The color contrast conductive layer may contain milled pigments. Thematerials are milled below 1 micron to form “nanopigments”. Suchpigments are effective in absorbing wavelengths of light in very thin or“sub micron” layers. In a preferred embodiment, the color contrastconductive layer absorbs all wavelengths of light across the visiblelight spectrum, that is, from 400 nanometers to 700 nanometerswavelength. For example, three different colorant pigments, such as ayellow pigment milled to median diameter of 120 nanometers, a magentapigment colorant milled to a median diameter of 210 nanometers, and acyan pigment colorant, such as Sunfast® Blue Pigment 15:4 pigment,milled to a median diameter of 110 nanometers are combined. A mixture ofthese three pigment colorants produces a uniform light absorption acrossthe visible spectrum. In order to ensure optimum packing, it ispreferred that the pigment colorants have a median particle diameter ofless than 50 percent of the thickness of the color contrast conductivelayer. Suitable nonconductive colorant pigments are readily availableand are designed to be light absorbing across the visible spectrum.Colorants for use in the present invention may also include dyecolorants, in addition to pigments. The preferred color of the pigmentor pigment combination is black, so that when incorporated into thecoating, it provides a high contrast background for an image in thedisplay.

Preferred pigments are the phthalocyanines such as Pigment Blue 15,15:1, 15:3, 15:4 and 15:6, anthraquinones such as Pigment Blue 60,quinacridones such as Pigment Red 122, Azos such as Pigment Yellow 74and Pigment Yellow 155, as listed in NPIRI Raw Materials Data Handbook,Vol. 4, Pigments, 1983, National Printing Research Institute. PigmentYellow 4G and Pigment Yellow 4G VP2532 are also useful. These pigmentcolorants are easily dispersed in an aqueous solution, and when combinedin certain proportions, have a hue sufficient to give an essentiallyneutral hue. Preferably, the dark layer in the display provides abackground that provides a substantially neutral optical density suchthat there is variability of less than +/−20% from the mean opticaldensity over at least 80% of the visible spectrum from 400 to 700 nm.

The carbon pigment colorant, for use in the present invention may be anycarbon-based black material. Preferably, the material is a“nanopigment”, preferably less than a micron in diameter, morepreferably less than 0.5 microns in diameter. Suitable carbon colorantsare referred to in the art as “carbon black.” For example, carbon blackCAS 001333-86-4 may be used. The ratio of carbon colorant to othercolorant(s) may vary from 100:0 to 0:100; however, it is preferred thatthe ratio varies from 100:0 to 75:25.

The colorants can be prepared by any suitable methods known in the art.Particularly relevant teachings are provided in U.S. Ser. No. 10/851,566and U.S. Pat. No. 6,788,362 for preparation of suitable colorantdispersions, and both incorporated herein by reference.

The color contrast conductive layer can be of any thickness. However,for the ease of manufacture the layer should be ≦2 μm (micron),preferably ≦1 μm (micron). In one embodiment, the thickness of the colorcontrast conductive layer is less than 25% of the said electricallymodulated imaging layer, which facilitates coating and drying inmanufacturing while maximizing the optical properties. The % visuallight transmission of the layer should be ≦20%, preferably ≦15%, andmore preferably ≦10%. The surface electrical resistivity (SER) of thecolor contrast conductive layer should be <10⁴ ohm/square, preferably<1500 ohm/square, more preferably <1000 ohm/square, and most preferably<500 ohm/square. The weight ratio of the electronically conductivepolymer to the colorant in the color contrast conductive layer of theinvention can vary from 99.9:0.1 to 0.1:99.9, but preferably from 95:5to 40:60, and most preferably from 80:20 to 50:50.

Visual light transmission value T of the color contrast conductive layeris determined from the total optical density at 530 nm, after correctingfor the contributions of the uncoated substrate. A Model 361T X-Ritedensitometer measuring total optical density at 530 nm, is best suitedfor this measurement.

Visual light transmission, T, is related to the corrected total opticaldensity at 530 nm, o.d.(corrected), by the following expression,T=1/(10^(o.d.(corrected)))

The SER value is typically determined by a standard four-pointelectrical probe.

In addition to the electronically conductive polymer and the colorant,the color contrast conductive layer of the invention may comprise afilm-forming binder to improve the physical properties of the layer. Insuch an embodiment, the layer may comprise from about 1 to 95% of thefilm-forming polymeric binder. However, the presence of the film formingbinder may increase the overall surface electrical resistivity of thelayer. The optimum weight percent of the film-forming polymer bindervaries depending on the electrical properties of the electronicallyconductive polymer, the chemical composition of the polymeric binder,and the requirements for the particular application.

Polymeric film-forming binders useful in this invention can include, butare not limited to, water-soluble or water-dispersible hydrophilicpolymers such as gelatin, gelatin derivatives, maleic acid or maleicanhydride copolymers, polystyrene sulfonates, cellulose derivatives,such as carboxymethyl cellulose, hydroxyethyl cellulose, celluloseacetate butyrate, diacetyl cellulose, and triacetyl cellulose,polyethylene oxide, polyvinyl alcohol, and poly-N-vinylpyrrolidone.Other suitable binders include aqueous emulsions of addition-typehomopolymers and copolymers prepared from ethylenically unsaturatedmonomers such as acrylates including acrylic acid, methacrylatesincluding methacrylic acid, acrylamides and methacrylamides, itaconicacid and its half-esters and diesters, styrenes including substitutedstyrenes, acrylonitrile and methacrylonitrile, vinyl acetates, vinylethers, vinyl and vinylidene halides, and olefins and aqueousdispersions of polyurethanes and polyesterionomers.

Other ingredients that may be included in the layer containing theelectronically conductive polymer include but are not limited tosurfactants, defoamers or coating aids, charge control agents,thickeners or viscosity modifiers, antiblocking agents, coalescing aids,crosslinking agents or hardeners, inorganic or polymeric particles,adhesion promoting agents, bite solvents or chemical etchants,lubricants, plasticizers, antioxidants, and other addenda that arewell-known in the art. Preferred bite solvents can include any of thevolatile aromatic compounds disclosed in U.S. Pat. No. 5,709,984,incorporated herein in by reference, as “conductivity-increasing”aromatic compounds, comprising an aromatic ring substituted with atleast one hydroxy group or a hydroxy substituted substituents group.These compounds include phenol, 4-chloro-3-methyl phenol,4-chlorophenol, 2-cyanophenol, 2,6-dichlorophenol, 2-ethylphenol,resorcinol, benzyl alcohol, 3-phenyl-1-propanol, 4-methoxyphenol,1,2-catechol, 2,4-dihydroxytoluene, 4-chloro-2-methyl phenol,2,4-dinitrophenol, 4-chlororesorcinol, 1-naphthol, 1,3-naphthalenedioland the like. These bite solvents are particularly suitable forpolyester based polymer sheets of the invention. Of this group, the mostpreferred compounds are resorcinol and 4-chloro-3-methyl phenol.Preferred surfactants suitable for these coatings include nonionic andanionic surfactants. Preferred cross-linking agents suitable for thesecoatings include silane compounds such as those disclosed in U.S. Pat.No. 5,370,981, incorporated herein in by reference.

For higher conductivities, the color contrast conductive layer mayadditionally comprise other materials such as silver (Ag) aluminum (Al),copper (Cu), nickel (Ni), cadmium (Cd), gold (Au), zinc (Zn), magnesium(Mg), tin (Sn), indium (In), tantalum (Ta), titanium (Ti), zirconium(Zr), cerium (Ce), silicon (Si), lead (Pb) or palladium (Pd). In apreferred embodiment, the color contrast conductive layer comprises atleast one of gold, silver and a gold/silver alloy, for example, a layerof silver coated on one or both sides with a thinner layer of gold. See,Int. Publ. No. WO 99/36261 by Polaroid Corporation, incorporated hereinin by reference. In another embodiment, the conductive layer maycomprise a layer of silver alloy, for example, a layer of silver coatedon one or both sides with a layer of indium cerium oxide (InCeO). SeeU.S. Pat. No. 5,667,853, incorporated herein in by reference.

The color contrast conductive layer may be formed by any method known inthe art. Such methods include air knife coating, gravure coating, hoppercoating, roller coating, spray coating, electrochemical coating, inkjetprinting, flexographic printing, screen printing, stamping and the like.Alternatively, the color contrast conductive layer can be transferred toa receiver member from a donor member by the application of heat and/orpressure, as disclosed in U.S. Ser. No. 10/969,889, incorporated hereinin by reference. An adhesive layer may be preferably present between thecolor contrast conductive layer and the receiver member.

Another preferred method of forming the color contrast conductive layeris by thermal transfer as disclosed in a series of US patents and patentapplications, e.g., U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520;6,221,553; 6,582,876; 6,586,153 by Wolk et al.; 6,610,455; 6,582,875;6,252,621; 2004/0029039 A1; by Tutt et al., 5,171,650 by Ellis et al.;2004/0065970 A1 by Blanchet-Fincher, all incorporated herein in byreference. Accordingly, it is envisioned that a thermal transfer elementcomprising a donor substrate and a multicomponent transfer unit can beformed wherein the multicomponent transfer unit comprises the colorcontrast conductive layer of the invention. Such a transfer unit isfully or partially transferred through the application of heat onto areceiver substrate, thus incorporating the color contrast conductivelayer of the invention on the receiver substrate.

The color contrast conductive layer may be patterned irradiating thelayer with ultraviolet (UV) or infrared (IR) radiation so that portionsof the color contrast conductive layer are ablated therefrom. Suchmethods are known in the art. For example, it is known to employ aninfrared (IR) fiber laser for patterning a metallic conductive layeroverlying a plastic film, directly ablating the conductive layer byscanning a pattern over the conductor/film structure. See: Int. Publ.No. WO 99/36261 and “42.2: A New Conductor Structure for Plastic LCDApplications Utilizing ‘All Dry’ Digital Laser Patterning,” 1998 SIDInternational Symposium Digest of Technical Papers, Anaheim, Calif., May17-22, 1998, no. VOL. 29, May 17, 1998, pages 1099-1101, bothincorporated herein by reference. Alternatively, the color contrastconductive layer may be patterned by pattern-wise deposition via inkjetprinting and/or any method disclosed in U.S. Ser. Nos. 10/944,570 and10/969,889, incorporated herein in by reference and references therein.

The color contrast conductive layer may be positioned between anelectrically imageable layer, such as a liquid crystal layer, and asubstrate. In this embodiment the display is viewed from a pointopposite the substrate. Alternatively, the color contrast conductivelayer may be positioned on the side of the electrically imageable layeropposite the substrate, for an embodiment where the display is viewedthrough the substrate. In both of these embodiments, a transparentconductive layer is provided on the viewing side of the electricallyimageable layer for the application of the necessary electrical fieldfor the display to operate.

These two embodiments are schematically illustrated in FIGS. 1 and 2 fora single color display. According to FIG. 1, substrate 10 is providedwith the color contrast conductive layer 12 of the invention, which isin contact with the electrically imageable layer 14. A transparentconductive layer 16 is placed on the electrically imageable layer 14.The display thus represented in FIG. 1 is suitable for viewing from theside opposite the substrate. According to FIG. 2, substrate 10 isprovided with a transparent conductive layer 16, which is in contactwith the electrically imageable layer 14. The color contrast conductivelayer 12 of the invention is placed on the electrically imageable layer14. The display thus represented in FIG. 2 is suitable for viewingthrough the substrate.

An embodiment of a multicolored display is schematically illustrated inFIG. 3. The display comprises substrate 35 that is provided with a colorcontrast conductive layer 55. A red imageable layer 25 is coated on topof the color contrast conductive layer. A layer of transparent conductor13 c is coated on top of the red imageable layer. A dielectric layer 41b is provided on top of the transparent conductor 13 c. A layer oftransparent conductor 17 b is coated on the dielectric layer 41 b. Agreen imageable layer 27 is coated on top of the of the transparentconductor 17 b. Another layer of transparent conductor 13 b is thenapplied on to the green imageable layer 27. Another dielectric layer 41a with the same composition as 41 b is coated on the transparentconductor 13 b. A transparent conductor 17 a is provided on top of thedielectric layer 41 a. A blue imageable layer 15 is next applied on thetransparent conductor 17 a. A final layer of a transparent conductor 13a is applied to the blue imageable layer 15. This display is viewed fromthe opposite side of the substrate. The substrate is not in the activeview plane of the display. In the above embodiment, red, green and blueimageable layers refer to imageable layers that when properly biasedreflect red, green and blue light, respectively.

The color contrast conductive layer of the invention can be formed onany rigid or flexible substrate. The substrates can be transparent,translucent or opaque, and may be colored or colorless. Rigid substratescan include glass, metal, ceramic and/or semiconductors. Flexiblesubstrates, especially those comprising a plastic substrate, arepreferred for their versatility and ease of manufacturing, coating andfinishing.

The flexible plastic substrate can be any flexible self-supportingplastic film that substrates the conductive polymeric film. “Plastic”means a high polymer, usually made from polymeric synthetic resins,which may be combined with other ingredients, such as curatives,fillers, reinforcing agents, colorants, and plasticizers. Plasticincludes thermoplastic materials and thermosetting materials.

The flexible plastic film must have sufficient thickness and mechanicalintegrity so as to be self-supporting, yet should not be so thick as tobe rigid. Another significant characteristic of the flexible plasticsubstrate material is its glass transition temperature (Tg). Tg isdefined as the glass transition temperature at which plastic materialwill change from the glassy state to the rubbery state. It may comprisea range before the material may actually flow. Suitable materials forthe flexible plastic substrate include thermoplastics of a relativelylow glass transition temperature, for example up to 150° C., as well asmaterials of a higher glass transition temperature, for example, above150° C. The choice of material for the flexible plastic substrate woulddepend on factors such as manufacturing process conditions, for example,deposition temperature, and annealing temperature, as well aspost-manufacturing conditions such as in a process line of a displaysmanufacturer. Certain of the plastic substrates discussed below canwithstand higher processing temperatures of up to at least about 200°C., some up to 300°-350° C., without damage.

Typically, the flexible plastic substrate can comprise any of thefollowing materials: polyester or polyester ionomer, polyethersulfone(PES), polycarbonate (PC), polysulfone, a phenolic resin, an epoxyresin, polyimide, polyetherester, polyetheramide, cellulose nitrate,cellulose acetate such as cellulose diacetate or cellulose triacetate,poly(vinyl acetate), polystyrene, polyolefins including polyolefinionomers, polyamide, aliphatic polyurethanes, polyacrylonitrile,polytetrafluoroethylenes, polyvinylidene fluorides, poly(methyl(x-methacrylates), an aliphatic or cyclic polyolefin, polyarylate (PAR),polyetherimide (PEI), polyethersulphone (PES), polyimide (PI), Teflonpoly(perfluoro-alboxy)fluoropolymer (PFA), poly(ether ether ketone)(PEEK), poly(ether ketone) (PEK), poly(ethylenetetrafluoroethylene)fluoropolymer (PETFE), poly(methyl methacrylate)(PMMA), various acrylate/methacrylate copolymers, natural or syntheticpaper, resin-coated or laminated paper, voided polymers includingpolymeric foam, microvoided polymers, microporous materials, fabric, orany combinations thereof.

Aliphatic polyolefins may include high density polyethylene (HDPE), lowdensity polyethylene (LDPE), and polypropylene, including orientedpolypropylene (OPP). Cyclic polyolefins may includepoly(bis(cyclopentadiene)). A preferred flexible plastic substrate is acyclic polyolefin or a polyester. Various cyclic polyolefins aresuitable for the flexible plastic substrate. Examples include Arton®made by Japan Synthetic Rubber Co., Tokyo, Japan; Zeanor T made by ZeonChemicals L.P., Tokyo Japan; and Topas® made by Celanese A. G., KronbergGermany. Arton is a poly(bis(cyclopentadiene)) condensate that is a filmof a polymer. Alternatively, the flexible plastic substrate can be apolyester. A preferred polyester is an aromatic polyester such asArylite. Although the substrate can be transparent, translucent oropaque, for most display applications transparent members comprisingtransparent substrate(s) are preferred. Although various examples ofplastic substrates are set forth above, it should be appreciated thatthe flexible substrate can also be formed from other materials such asflexible glass and ceramic.

The flexible plastic substrate can be reinforced with a hard coating.Typically, the hard coating is an acrylic coating. Such a hard coatingtypically has a thickness of from 1 to 15 microns, preferably from 2 to4 microns and can be provided by free radical polymerization, initiatedeither thermally or by ultraviolet radiation, of an appropriatepolymerizable material. Depending on the substrate, different hardcoatings can be used. When the substrate is polyester or Arton, aparticularly preferred hard coating is the coating known as “Lintec.”Lintec contains UV cured polyester acrylate and colloidal silica. Whendeposited on Arton, it has a surface composition of 35 atom % C, 45 atom% 0, and 20 atom % Si, excluding hydrogen. Another particularlypreferred hard coating is the acrylic coating sold under the trademark“Terrapin” by Tekra Corporation, New Berlin, Wis.

The most preferred flexible plastic substrate is a polyester because ofits superior mechanical and thermal properties as well as itsavailability in large quantity at a moderate price. From an opticalperformance perspective, polymers such as cellulose acetate are highlypreferred because of their low birefringence.

The particular polyester chosen for use can be a homo-polyester or aco-polyester, or mixtures thereof as desired. The polyester can becrystalline or amorphous or mixtures thereof as desired. Polyesters arenormally prepared by the condensation of an organic dicarboxylic acidand an organic diol and, therefore, illustrative examples of usefulpolyesters will be described herein below in terms of these diol anddicarboxylic acid precursors.

Polyesters which are suitable for use in this invention are those whichare derived from the condensation of aromatic, cycloaliphatic, andaliphatic diols with aliphatic, aromatic and cycloaliphatic dicarboxylicacids and may be cycloaliphatic, aliphatic or aromatic polyesters.Exemplary of useful cycloaliphatic, aliphatic and aromatic polyesterswhich can be utilized in the practice of their invention arepoly(ethylene terephthalate), poly(cyclohexlenedimethylene),terephthalate) poly(ethylene dodecate), poly(butylene terephthalate),poly(ethylene naphthalate), poly(ethylene(2,7-naphthalate)),poly(methaphenylene isophthalate), poly(glycolic acid), poly(ethylenesuccinate), poly(ethylene adipate), poly(ethylene sebacate),poly(decamethylene azelate), poly(ethylene sebacate), poly(decamethyleneadipate), poly(decamethylene sebacate), poly(dimethylpropiolactone),poly(para-hydroxybenzoate) (Ekonol), poly(ethylene oxybenzoate)(A-tell), poly(ethylene isophthalate), poly(tetramethyleneterephthalate, poly(hexamethylene terephthalate), poly(decamethyleneterephthalate), poly(1,4-cyclohexane dimethylene terephthalate) (trans),poly(ethylene 1,5-naphthalate), poly(ethylene 2,6-naphthalate),poly(1,4-cyclohexylene dimethylene terephthalate), (Kodel) (cis), andpoly(1,4-cyclohexylene dimethylene terephthalate (Kodel) (trans).

Polyester compounds prepared from the condensation of a diol and anaromatic dicarboxylic acid is preferred for use in this invention.Illustrative of such useful aromatic carboxylic acids are terephthalicacid, isophthalic acid and an α-phthalic acid,1,3-napthalenedicarboxylic acid, 1,4 napthalenedicarboxylic acid,2,6-napthalenedicarboxylic acid, 2,7-napthalenedicarboxylic acid,4,4′-diphenyldicarboxylic acid, 4,4′-diphenysulfphone-dicarboxylic acid,1,1,3-trimethyl-5-carboxy-3-(p-carboxyphenyl)-idane, diphenyl ether4,4′-dicarboxylic acid, bis-p(carboxy-phenyl)methane, and the like. Ofthe aromatic dicarboxylic acids, those based on a benzene ring (such asterephthalic acid, isophthalic acid, orthophthalic acid) are preferredfor use in the practice of this invention. Amongst these preferred acidprecursors, terephthalic acid is particularly preferred acid precursor.

Preferred polyesters for use in the practice of this invention includepoly(ethylene terephthalate), poly(butylene terephthalate),poly(1,4-cyclohexylene dimethylene terephthalate) and poly(ethylenenaphthalate) and copolymers and/or mixtures thereof. Among thesepolyesters of choice, poly(ethylene terephthalate) is most preferred.

Most preferred cellulose acetate for use in the present invention iscellulose triacetate, also known as triacetylcellulose or TAC. TAC filmhas traditionally been used by the photographic industry due to itsunique physical properties, and flame retardance. TAC film is also thepreferred polymer film for use as a cover sheet for polarizers used inliquid crystal displays.

The manufacture of TAC films by a casting process is well known andincludes the following process. A TAC solution in organic solvent (dope)is typically cast on a drum or a band, and the solvent is evaporated toform a film. Before casting the dope, the concentration of the dope istypically so adjusted that the solid content of the dope is in the rangeof 18 to 35 wt. %. The surface of the drum or band is typically polishedto give a mirror plane. The casting and drying stages of the solventcast methods are described in U.S. Pat. Nos. 2,336,310, 2, 367,603,2,492,078, 2,492,977, 2,492,978, 2,607,704, 2,739,069, 2,739,070,British Patent Nos. 640,731, 736,892, Japanese Patent Publication Nos.45(1970)-4554, 49(1974)-5614, Japanese Patent Provisional PublicationNos. 60(1985)-176834, 60(1985)-203430 and 62(1987)-115035.

A plasticizer can be added to the cellulose acetate film to improve themechanical strength of the film. The plasticizer has another function ofshortening the time for the drying process. Phosphoric esters andcarboxylic esters (such as phthalic esters and citric esters) areusually used as the plasticizer. Examples of the phosphoric estersinclude triphenyl phosphate (TPP) and tricresyl phosphate (TCP).Examples of the phthalic esters include dimethyl phthalate (DMP),diethyl phthalate (DEP), dibutyl phthalate (DBP), dioctyl phthalate(DOP), diphenyl phthalate (DPP) and diethylhexyl phthalate (DEHP).Examples of the citric esters include o-acetyltriethyl citrate (OACTE)and o-acetyltributyl citrate (OACTB). The amount of the plasticizer isin the range of typically 0.1 to 25 wt. %, conveniently 1 to 20 wt. %,desirably 3 to 15 wt. % based on the amount of cellulose acetate.

The substrate useful for application in display devices can be planarand/or curved. The curvature of the substrate can be characterized by aradius of curvature, which may have any value. Alternatively, thesubstrate may be bent so as to form an angle. This angle may be anyangle from 0° to 360°, including all angles therebetween and all rangestherebetween. If the substrate is electrically conducting, an insulatingmaterial such as a non-conductive polymer may be placed between thesubstrate and the conducting polymer.

The substrate may be of any thickness, such as, for example 10⁻⁸ cm to 1cm including all values in between and all ranges therebetween. Thickerand thinner layers may be used. The substrate need not have a uniformthickness. The preferred shape is square or rectangular, although anyshape may be used. Before the substrate is coated with the conductingpolymer it may be physically and/or optically patterned, for example, byrubbing, by the application of an image, by the application of patternedelectrical contact areas, by the presence of one or more colors indistinct regions, by embossing, microembossing, and microreplication.

The substrate can comprise a single layer or multiple layers accordingto need. The multiplicity of layers may include any number of auxiliarylayers such as antistatic layers, tie layers or adhesion promotinglayers, abrasion resistant layers, curl control layers, conveyancelayers, barrier layers, splice providing layers, UV absorption layers,optical effect providing layers, such as antireflective and antiglarelayers, waterproofing layers, adhesive layers, imaging layers and thelike.

The polymer substrate may be formed by any method known in the art suchas those involving extrusion, coextrusion, quenching, orientation, heatsetting, lamination, coating and solvent casting. It is preferred thatthe polymer substrate is an oriented sheet formed by any suitable methodknown in the art, such as by a flat sheet process or a bubble or tubularprocess. The flat sheet process involves extruding or coextruding thematerials of the sheet through a slit die and rapidly quenching theextruded or coextruded web upon a chilled casting drum so that thepolymeric component(s) of the sheet are quenched below theirsolidification temperature.

The quenched sheet is then biaxially oriented by stretching in mutuallyperpendicular directions at a temperature above the glass transitiontemperature of the polymer(s). The sheet may be stretched in onedirection and then in a second direction or may be simultaneouslystretched in both directions. The preferred stretch ratio in anydirection is at least 3:1. After the sheet has been stretched, it isheat set by heating to a temperature sufficient to crystallize thepolymers while restraining to some degree the sheet against retractionin both directions of stretching.

The polymer sheet may be subjected to any number of coatings andtreatments, after extrusion, coextrusion, and orientation or betweencasting and full orientation, to improve its properties, such asprintability, barrier properties, heat-sealability, spliceability,adhesion to other substrates and/or imaging layers. Examples of suchcoatings include acrylic coatings for printability, and polyvinylidenehalide for heat seal properties. Examples of such treatments may includeflame, plasma and corona discharge treatment, ultraviolet radiationtreatment, ozone treatment and electron beam treatment to improvecoatability and adhesion. Further examples of treatments may becalendaring, embossing and patterning to obtain specific effects on thesurface of the web. The polymer sheet can be further incorporated in anyother suitable substrate by lamination, adhesion, cold or heat sealing,extrusion coating, or any other method known in the art.

In one embodiment, at least one imageable layer is applied to thesupport. The imageable layer can contain an electrically imageablematerial. The electrically imageable material can be light emitting orlight modulating. Light emitting materials can be inorganic or organicin nature. Particularly preferred are organic light emitting diodes(OLED) or polymeric light emitting diodes (PLED). The light modulatingmaterial can be reflective or transmissive. Light modulating materialscan be electrochemical, electrophoretic, such as Gyricon particles,electrochromic, or liquid crystals. The liquid crystalline material canbe twisted nematic (TN), super-twisted nematic (STN), ferroelectric,magnetic, or chiral nematic liquid crystals. Especially preferred arechiral nematic liquid crystals. The chiral nematic liquid crystals canbe polymer dispersed liquid crystals (PDLC). Structures having stackedimaging layers or multiple support layers, however, are optional forproviding additional advantages in some case.

In a preferred embodiment, the electrically imageable material can beaddressed with an electric field and then retain its image after theelectric field is removed, a property typically referred to as“bistable”. Particularly suitable electrically imageable materials thatexhibit “bistability” are electrochemical, electrophoretic, such asGyricon particles, electrochromic, magnetic, or chiral nematic liquidcrystals. Especially preferred are chiral nematic liquid crystals. Thechiral nematic liquid crystals can be polymer dispersed liquid crystals(PDLC).

The electrically modulated material may also be a printable, conductiveink having an arrangement of particles or microscopic containers ormicrocapsules. Each microcapsule contains an electrophoretic compositionof a fluid, such as a dielectric or emulsion fluid, and a suspension ofcolored or charged particles or colloidal material. The diameter of themicrocapsules typically ranges from about 30 to about 300 microns.According to one practice, the particles visually contrast with thedielectric fluid. According to another example, the electricallymodulated material may include rotatable balls that can rotate to exposea different colored surface area, and which can migrate between aforward viewing position and/or a rear nonviewing position, such asgyricon. Specifically, gyricon is a material comprised of twistingrotating elements contained in liquid filled spherical cavities andembedded in an elastomer medium. The rotating elements may be made toexhibit changes in optical properties by the imposition of an externalelectric field. Upon application of an electric field of a givenpolarity, one segment of a rotating element rotates toward, and isvisible by an observer of the display. Application of an electric fieldof opposite polarity, causes the element to rotate and expose a second,different segment to the observer. A gyricon display maintains a givenconfiguration until an electric field is actively applied to the displayassembly. Gyricon particles typically have a diameter of about 100microns. Gyricon materials are disclosed in U.S. Pat. No. 6,147,791,U.S. Pat. No. 4,126,854 and U.S. Pat. No. 6,055,091, the contents ofwhich are herein incorporated by reference.

According to one practice, the microcapsules may be filled withelectrically charged white particles in a black or colored dye. Examplesof electrically modulated material and methods of fabricating assembliescapable of controlling or effecting the orientation of the ink suitablefor use with the present invention are set forth in International PatentApplication Publication Number WO 98/41899, International PatentApplication Publication Number WO 98/19208, International PatentApplication Publication Number WO 98/03896, and International PatentApplication Publication Number WO 98/41898, the contents of which areherein incorporated by reference.

The electrically modulated material may also include material disclosedin U.S. Pat. No. 6,025,896, the contents of which are incorporatedherein by reference. This material comprises charged particles in aliquid dispersion medium encapsulated in a large number ofmicrocapsules. The charged particles can have different types of colorand charge polarity. For example white positively charged particles canbe employed along with black negatively charged particles. The describedmicrocapsules are disposed between a pair of electrodes, such that adesired image is formed and displayed by the material by varying thedispersion state of the charged particles. The dispersion state of thecharged particles is varied through a controlled electric field appliedto the electrically modulated material. According to a preferredembodiment, the particle diameters of the microcapsules are betweenabout 5 microns and about 200 microns, and the particle diameters of thecharged particles are between about one-thousandth and one-fifth thesize of the particle diameters of the microcapsules.

Further, the electrically modulated material may include a thermochromicmaterial. A thermochromic material is capable of changing its statealternately between transparent and opaque upon the application of heat.In this manner, a thermochromic imaging material develops images throughthe application of heat at specific pixel locations in order to form animage. The thermochromic imaging material retains a particular imageuntil heat is again applied to the material. Since the rewritablematerial is transparent, UV fluorescent printings, designs and patternsunderneath can be seen through.

The electrically modulated material may also include surface stabilizedferroelectric liquid crystals (SSFLC). Surface stabilized ferroelectricliquid crystals confining ferroelectric liquid crystal material betweenclosely spaced glass plates to suppress the natural helix configurationof the crystals. The cells switch rapidly between two opticallydistinct, stable states simply by alternating the sign of an appliedelectric field.

Magnetic particles suspended in an emulsion comprise an additionalimaging material suitable for use with the present invention.Application of a magnetic force alters pixels formed with the magneticparticles in order to create, update or change human and/or machinereadable indicia. Those skilled in the art will recognize that a varietyof bistable nonvolatile imaging materials are available and may beimplemented in the present invention.

The electrically modulated material may also be configured as a singlecolor, such as black, white or clear, and may be fluorescent,iridescent, bioluminescent, incandescent, ultraviolet, infrared, or mayinclude a wavelength specific radiation absorbing or emitting material.There may be multiple layers of electrically modulated material.Different layers or regions of the electrically modulated materialdisplay material may have different properties or colors. Moreover, thecharacteristics of the various layers may be different from each other.For example, one layer can be used to view or display information in thevisible light range, while a second layer responds to or emitsultraviolet light. The nonvisible layers may alternatively beconstructed of non-electrically modulated material based materials thathave the previously listed radiation absorbing or emittingcharacteristics. The electrically modulated material employed inconnection with the present invention preferably has the characteristicthat it does not require power to maintain display of indicia.

Most preferred is a support bearing a conventional polymer dispersedlight modulating material. The liquid crystal (LC) is used as an opticalswitch. The LC material is provided with two conductive electrodes toinduce an electric field, which can cause a phase change or state changein the LC material, the LC exhibiting different light reflectingcharacteristics according to its phase and/or state.

As used herein, a “liquid crystal display” (LCD) is a type of flat paneldisplay used in various electronic devices. At a minimum, an LCDcomprises a substrate, at least one conductive layer and a liquidcrystal layer. LCDs may also comprise two sheets of polarizing materialwith a liquid crystal solution between the polarizing sheets. The sheetsof polarizing material may comprise a substrate of glass or transparentplastic. The LCD may also include functional layers.

In a preferred embodiment, the electrically imageable layer can comprisechiral nematic liquid crystal. In the fully evolved focal conic state,the chiral nematic liquid crystal is transparent, passing incidentlight, which is absorbed by the light absorber to create a black image.Progressive evolution of the focal conic state causes a viewer toperceive a reflected light that transitions to black as the chiralnematic material changes from planar state to a focal conic state. Thetransition to the light transmitting state is progressive, and varyingthe low voltage time permits variable levels of reflection. Thesevariable levels may be mapped out to corresponding gray levels, and whenthe field is removed, the light modulating layer maintains a givenoptical state indefinitely. This process is more fully discussed in U.S.Pat. No. 5,437,811, incorporated herein by reference.

Liquid crystals can be nematic (N), chiral nematic (N*), or smectic,depending upon the arrangement of the molecules in the mesophase. Chiralnematic liquid crystal (N*LC) displays are typically reflective, thatis, no backlight is needed, and can function without the use ofpolarizing films or a color filter.

Chiral nematic liquid crystal refers to the type of liquid crystalhaving finer pitch than that of twisted nematic and super-twistednematic used in commonly encountered LC devices. Chiral nematic liquidcrystals are so named because such liquid crystal formulations arecommonly obtained by adding chiral agents to host nematic liquidcrystals. Chiral nematic liquid crystals may be used to produce bistableor multi-stable displays. These devices have significantly reduced powerconsumption due to their nonvolatile “memory” characteristic. Since suchdisplays do not require a continuous driving circuit to maintain animage, they consume significantly reduced power. Chiral nematic displaysare bistable in the absence of a field; the two stable textures are thereflective planar texture and the weakly scattering focal conic texture.In the planar texture, the helical axes of the chiral nematic liquidcrystal molecules are substantially perpendicular to the substrate uponwhich the liquid crystal is disposed. In the focal conic state thehelical axes of the liquid crystal molecules are generally randomlyoriented. Adjusting the concentration of chiral dopants in the chiralnematic material modulates the pitch length of the mesophase and, thus,the wavelength of radiation reflected. Chiral nematic materials thatreflect infrared radiation and ultraviolet have been used for purposesof scientific study. Commercial displays are most often fabricated fromchiral nematic materials that reflect visible light. Some known LCDdevices include chemically etched, transparent, conductive layersoverlying a glass substrate as described in U.S. Pat. No. 5,667,853,incorporated herein by reference.

In one embodiment, a chiral nematic liquid crystal composition may bedispersed in a continuous matrix. Such materials are referred to as“polymer dispersed liquid crystal” materials or “PDLC” materials. Suchmaterials can be made by a variety of methods. For example, Doane et al.(Applied Physics Letters, 48, 269 (1986)) disclose a PDLC comprisingapproximately 0.4 μm droplets of nematic liquid crystal 5CB in a polymerbinder. A phase separation method is used for preparing the PDLC. Asolution containing monomer and liquid crystal is filled in a displaycell and the material is then polymerized. Upon polymerization theliquid crystal becomes immiscible and nucleates to form droplets. Westet al. (Applied Physics Letters 63, 1471 (1993)) disclose a PDLCcomprising a chiral nematic mixture in a polymer binder. Once again aphase separation method is used for preparing the PDLC. The liquidcrystal material and polymer, preferably a hydroxy functionalizedpolymethylmethacrylate, along with a crosslinker for the polymer aredissolved in a common organic solvent toluene and coated on an indiumtin oxide (ITO) substrate. A dispersion of the liquid crystal materialin the polymer binder is formed upon evaporation of toluene at hightemperature. The phase separation methods of Doane et al. and West etal. require the use of organic solvents that may be objectionable incertain manufacturing environments.

The contrast of the display is degraded if there is more than asubstantial monolayer of N*LC domains. The term “substantial monolayer”is defined by the Applicants to mean that, in a direction perpendicularto the plane of the display, there is no more than a single layer ofdomains sandwiched between the electrodes at most points of the displayor the imaging layer, preferably at 75 percent or more of the points orarea of the display, most preferably at 90 percent or more of the pointsor area of the display. In other words, at most, only a minor portion,that is, preferably less than 10 percent of the points or area of thedisplay has more than a single domain, that is, two or more domains,between the electrodes in a direction perpendicular to the plane of thedisplay, compared to the amount of points or area of the display atwhich there is only a single domain between the electrodes.

The amount of material needed for a monolayer can be accuratelydetermined by calculation based on individual domain size, assuming afully closed packed arrangement of domains. In practice, there may beimperfections in which gaps occur and some unevenness due to overlappingdroplets or domains. On this basis, the calculated amount is preferablyless than about 150 percent of the amount needed for monolayer domaincoverage, preferably not more than about 125 percent of the amountneeded for a monolayer domain coverage, more preferably not more than110 percent of the amount needed for a monolayer of domains.Furthermore, improved viewing angle and broadband features may beobtained by appropriate choice of differently doped domains based on thegeometry of the coated droplet and the Bragg reflection condition.

In a preferred embodiment of the invention, the display device ordisplay sheet has simply a single imaging layer of liquid crystalmaterial along a line perpendicular to the face of the display,preferably a single layer coated on a flexible substrate. Such asstructure, as compared to vertically stacked imaging layers each betweenopposing substrates, is especially advantageous for monochrome shelflabels and the like. Structures having stacked imaging layers, however,are optional for providing additional advantages in some case.

Preferably, the domains are flattened spheres and have on average athickness substantially less than their length, preferably at least 50%less. More preferably, the domains on average have a thickness (depth)to length ratio of 1:2 to 1:6. The flattening of the domains can beachieved by proper formulation and sufficiently rapid drying of thecoating. The domains preferably have an average diameter of 2 to 30microns. The imaging layer preferably has a thickness of 10 to 150microns when first coated and 2 to 20 microns when dried.

The flattened domains of liquid crystal material can be defined ashaving a major axis and a minor axis. In a preferred embodiment of adisplay or display sheet, the major axis is larger in size than the cellor imaging layer thickness for a majority of the domains. Such adimensional relationship is shown in U.S. Pat. No. 6,061,107, herebyincorporated by reference in its entirety.

There are alternative display technologies to LCDs that can be used, forexample, in flat panel displays. A notable example is organic or polymerlight emitting devices (OLEDs) or (PLEDs), which are comprised ofseveral layers in which one of the layers is comprised of an organicmaterial that can be made to electroluminesce by applying a voltageacross the device. An OLED device is typically a laminate formed in asubstrate such as glass or a plastic polymer. A light emitting layer ofa luminescent organic solid, as well as adjacent semiconductor layers,are sandwiched between an anode and a cathode. The semiconductor layerscan be hole injecting and electron injecting layers. PLEDs can beconsidered a subspecies of OLEDs in which the luminescent organicmaterial is a polymer. The light emitting layers may be selected fromany of a multitude of light emitting organic solids, e.g., polymers thatare suitably fluorescent or chemiluminescent organic compounds. Suchcompounds and polymers include metal ion salts of 8-hydroxyquinolate,trivalent metal quinolate complexes, trivalent metal bridged quinolatecomplexes, Schiff-based divalent metal complexes, tin (IV) metalcomplexes, metal acetylacetonate complexes, metal bidenate ligandcomplexes incorporating organic ligands, such as 2-picolylketones,2-quinaldylketones, or 2-(o-phenoxy) pyridine ketones, bisphosphonates,divalent metal maleonitriledithiolate complexes, molecular chargetransfer complexes, rare earth mixed chelates, (5-hydroxy) quinoxalinemetal complexes, aluminum tris-quinolates, and polymers such aspoly(p-phenylenevinylene), poly(dialkoxyphenylenevinylene),poly(thiophene), poly(fluorene), poly(phenylene), poly(phenylacetylene),poly(aniline), poly(3-alkylthiophene), poly(3-octylthiophene), andpoly(N-vinylcarbazole). When a potential difference is applied acrossthe cathode and anode, electrons from the electron injecting layer andholes from the hole injecting layer are injected into the light emittinglayer; they recombine, emitting light. OLEDs and PLEDs are described inthe following United States patents, all of which are incorporatedherein by this reference: U.S. Pat. No. 5,707,745 to Forrest et al.,U.S. Pat. No. 5,721,160 to Forrest et al., U.S. Pat. No. 5,757,026 toForrest et al., U.S. Pat. No. 5,834,893 to Bulovic et al., U.S. Pat. No.5,861,219 to Thompson et al., U.S. Pat. No. 5,904,916 to Tang et al.,U.S. Pat. No. 5,986,401 to Thompson et al., U.S. Pat. No. 5,998,803 toForrest et al., U.S. Pat. No. 6,013,538 to Burrows et al., U.S. Pat. No.6,046,543 to Bulovic et al., U.S. Pat. No. 6,048,573 to Tang et al.,U.S. Pat. No. 6,048,630 to Burrows et al., U.S. Pat. No. 6,066,357 toTang et al., U.S. Pat. No. 6,125,226 to Forrest et al., U.S. Pat. No.6,137,223 to Hung et al., U.S. Pat. No. 6,242,115 to Thompson et al.,and U.S. Pat. No. 6,274,980 to Burrows et al.

Modern chiral nematic liquid crystal materials usually include at leastone nematic host combined with a chiral dopant. In general, the nematicliquid crystal phase is composed of one or more mesogenic componentscombined to provide useful composite properties. Many such materials areavailable commercially. The nematic component of the chiral nematicliquid crystal mixture may be comprised of any suitable nematic liquidcrystal mixture or composition having appropriate liquid crystalcharacteristics. Nematic liquid crystals suitable for use in the presentinvention are preferably composed of compounds of low molecular weightselected from nematic or nematogenic substances, for example from theknown classes of the azoxybenzenes, benzylideneanilines, biphenyls,terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl estersof cyclohexanecarboxylic acid; phenyl or cyclohexyl esters ofcyclohexylbenzoic acid; phenyl or cyclohexyl esters ofcyclohexylcyclohexanecarboxylic acid; cyclohexylphenyl esters of benzoicacid, of cyclohexanecarboxylic acid and ofcyclohexylcyclohexanecarboxylic acid; phenyl cyclohexanes;cyclohexylbiphenyls; phenyl cyclohexylcyclohexanes;cyclohexylcyclohexanes; cyclohexylcyclohexenes;cyclohexylcyclohexylcyclohexenes; 1,4-bis-cyclohexylbenzenes;4,4-bis-cyclohexylbiphenyls; phenyl- or cyclohexylpyrimidines; phenyl-or cyclohexylpyridines; phenyl- or cyclohexylpyridazines; phenyl- orcyclohexyldioxanes; phenyl- or cyclohexyl-1,3-dithianes;1,2-diphenylethanes; 1,2-dicyclohexylethanes;1-phenyl-2-cyclohexylethanes;1-cyclohexyl-2-(4-phenylcyclohexyl)ethanes;1-cyclohexyl-2′,2-biphenylethanes; 1-phenyl-2-cyclohexylphenylethanes;optionally halogenated stilbenes; benzyl phenyl ethers; tolanes;substituted cinnamic acids and esters; and further classes of nematic ornematogenic substances. The 1,4-phenylene groups in these compounds mayalso be laterally mono- or difluorinated. The liquid crystallinematerial of this preferred embodiment is based on the achiral compoundsof this type. The most important compounds, that are possible ascomponents of these liquid crystalline materials, can be characterizedby the following formula R′—X—Y—Z—R″ wherein X and Z, which may beidentical or different, are in each case, independently from oneanother, a bivalent radical from the group formed by -Phe-, -Cyc-,-Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, —B-Phe- and —B-Cyc-;wherein Phe is unsubstituted or fluorine substituted 1,4-phenylene, Cycis trans-1,4-cyclohexylene or 1,4-cyclohexenylene, Pyr ispyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5-diyl,and B is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5-diyl,pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl. Y in these compounds isselected from the following bivalent groups —CH═CH—, —C≡C—, —N═N(O)—,—CH═CY′—, —CH═N(O)—, —CH2—CH2-, —CO—O—, —CH2—O—, —CO—S—, —CH2—S—,—COO-Phe-COO— or a single bond, with Y′ being halogen, preferablychlorine, or —CN; R′ and R″ are, in each case, independently of oneanother, alkyl, alkenyl, alkoxy, alkenyloxy, alkanoyloxy, alkoxycarbonylor alkoxycarbonyloxy with 1 to 18, preferably 1 to 12 C atoms, oralternatively one of R′ and R″ is —F, —CF3, —OCF3, —Cl, —NCS or —CN. Inmost of these compounds R′ and R′ are, in each case, independently ofeach another, alkyl, alkenyl or alkoxy with different chain length,wherein the sum of C atoms in nematic media generally is between 2 and9, preferably between 2 and 7. The nematic liquid crystal phasestypically consist of 2 to 20, preferably 2 to 15 components. The abovelist of materials is not intended to be exhaustive or limiting. Thelists disclose a variety of representative materials suitable for use ormixtures, which comprise the active element in electro-optic liquidcrystal compositions.

Suitable chiral nematic liquid crystal compositions preferably have apositive dielectric anisotropy and include chiral material in an amounteffective to form focal conic and twisted planar textures. Chiralnematic liquid crystal materials are preferred because of theirexcellent reflective characteristics, bistability and gray scale memory.The chiral nematic liquid crystal is typically a mixture of nematicliquid crystal and chiral material in an amount sufficient to producethe desired pitch length. Suitable commercial nematic liquid crystalsinclude, for example, E7, E44, E48, E31, E80, BL087, BL101, ZLI-3308,ZLI-3273, ZLI-5048-000, ZLI-5049-100, ZLI-5100-100, ZLI-5800-000,MLC-6041-100.TL202, TL203, TL204 and TL205 manufactured by E. Merck(Darrnstadt, Germany). Although nematic liquid crystals having positivedielectric anisotropy, and especially cyanobiphenyls, are preferred,virtually any nematic liquid crystal known in the art, including thosehaving negative dielectric anisotropy should be suitable for use in theinvention. Other nematic materials may also be suitable for use in thepresent invention as would be appreciated by those skilled in the art.

The chiral dopant added to the nematic mixture to induce the helicaltwisting of the mesophase, thereby allowing reflection of visible light,can be of any useful structural class. The choice of dopant depends uponseveral characteristics including among others its chemicalcompatibility with the nematic host, helical twisting power, temperaturesensitivity, and light fastness. Many chiral dopant classes are known inthe art: e.g., G. Gottarelli and G. Spada, Mol. Cryst. Liq. Crys., 123,377 (1985); G. Spada and G. Proni, Enantiomer, 3, 301 (1998) andreferences therein. Typical well known dopant classes include1,1-binaphthol derivatives; isosorbide (D-1) and similar isomannideesters as disclosed in U.S. Pat. No. 6,217,792; TADDOL derivatives (D-2)as disclosed in U.S. Pat. No. 6,099,751; and the pending spiroindanesesters (D-3) as disclosed in U.S. patent application Ser. No. 10/651,692by T. Welter et al., filed Aug. 29, 2003, titled “Chiral Compounds AndCompositions Containing The Same,” hereby incorporated by reference.

The pitch length of the liquid crystal materials may be adjusted basedupon the following equation (1):λ_(max)=n_(av)p₀where λ_(max) is the peak reflection wavelength, that is, the wavelengthat which reflectance is a maximum, n_(av) is the average index ofrefraction of the liquid crystal material, and p₀ is the natural pitchlength of the chiral nematic helix. Definitions of chiral nematic helixand pitch length and methods of its measurement, are known to thoseskilled in the art such as can be found in the book, Blinov, L. M.,Electro-optical and Magneto-Optical Properties of Liquid Crystals, JohnWiley & Sons Ltd. 1983. The pitch length is modified by adjusting theconcentration of the chiral material in the liquid crystal material. Formost concentrations of chiral dopants, the pitch length induced by thedopant is inversely proportional to the concentration of the dopant. Theproportionality constant is given by the following equation (2):p ₀=1/(HTP.c)where c is the concentration of the chiral dopant and HTP is theproportionality constant.

For some applications, it is desired to have LC mixtures that exhibit astrong helical twist and thereby a short pitch length. For example inliquid crystalline mixtures that are used in selectively reflectingchiral nematic displays, the pitch has to be selected such that themaximum of the wavelength reflected by the chiral nematic helix is inthe range of visible light. Other possible applications are polymerfilms with a chiral liquid crystalline phase for optical elements, suchas chiral nematic broadband polarizers, filter arrays, or chiral liquidcrystalline retardation films. Among these are active and passiveoptical elements or color filters and liquid crystal displays, forexample STN, TN, AMD-TN, temperature compensation, polymer free orpolymer stabilized chiral nematic texture (PFCT, PSCT) displays.Possible display industry applications include ultralight, flexible, andinexpensive displays for notebook and desktop computers, instrumentpanels, video game machines, videophones, mobile phones, hand held PCs,PDAs, e-books, camcorders, satellite navigation systems, store andsupermarket pricing systems, highway signs, informational displays,smart cards, toys, and other electronic devices.

Chiral nematic liquid crystal materials and cells, as well as polymerstabilized chiral nematic liquid crystals and cells, are well known inthe art and described in, for example, U.S. application Ser. No.07/969,093 and Ser. No. 08/057,662; Yang et al., Appl. Phys. Lett.60(25) pp 3102-04 (1992); Yang et al., J. Appl. Phys. 76(2) pp 1331(1994); published International Patent Application No. PCT/US92/09367;and published International Patent Application No. PCT/US92/03504, allof which are incorporated herein by reference.

In a preferred embodiment, a light modulating layer is deposited over afirst conductor. The light modulating layer contains a chiral nematicliquid crystal. The selected material preferably exhibits high opticaland electrical anisotropy and matches the index of refraction of thecarrier polymer, when the material is electrically oriented. Examples ofsuch materials are E. Merck's BL-03, BL-048 or BL-033, which areavailable from EM Industries of Hawthorne, N.Y. Other light reflectingor diffusing modulating, electrically operated materials can also becoated, such as a micro-encapsulated electrophoretic material in oil.

The liquid crystal can be a chiral doped nematic liquid crystal, alsoknown as cholesteric liquid crystal, such as those disclosed in U.S.Pat. No. 5,695,682. Application of fields of various intensity andduration change the state of chiral doped nematic materials from areflective to a transmissive state. These materials have the advantageof maintaining a given state indefinitely after the field is removed.Cholesteric liquid crystal materials can be Merck BL112, BL118 or BL126that are available from EM Industries of Hawthorne, N.Y. The lightmodulating layer is effective in two conditions.

Liquid crystal domains may be preferably made using a limitedcoalescence methodology, as disclosed in U.S. Pat. Nos. 6,556,262 and6,423,368, incorporated herein by reference. Limited coalescence isdefined as dispersing a light modulating material below a given size,and using coalescent limiting material to limit the size of theresulting domains. Such materials are characterized as having a ratio ofmaximum to minimum domain size of less than 2:1. By use of the term“uniform domains”, it is meant that domains are formed having a domainsize variation of less than 2:1. Limited domain materials have improvedoptical properties.

An immiscible, field responsive light modulating material along with aquantity of colloidal particles is dispersed in an aqueous system andblended to form a dispersion of field responsive, light modulatingmaterial below a coalescence size. When the dispersion, also referred toherein as an emulsion, is dried, a coated material is produced which hasa set of uniform domains having a plurality of electrically responsiveoptical states. The colloidal solid particle, functioning as anemulsifier, limits domain growth from a highly dispersed state.Uniformly sized liquid crystal domains are created and machine coated tomanufacture light modulating, electrically responsive sheets withimproved optical efficiency.

Specifically, a liquid crystal material may be dispersed an aqueous bathcontaining a water soluble binder material such as deionized gelatin,polyvinyl alcohol (PVA) or polyethylene oxide (PEO). Such compounds aremachine coatable on equipment associated with photographic films.Preferably, the binder has a low ionic content, as the presence of ionsin such a binder hinders the development of an electrical field acrossthe dispersed liquid crystal material. Additionally, ions in the bindercan migrate in the presence of an electrical field, chemically damagingthe light modulating layer. The liquid crystal/gelatin emulsion iscoated to a thickness of between 5 and 30 microns to optimize opticalproperties of light modulating layer. The coating thickness, size of theliquid crystal domains, and concentration of the domains of liquidcrystal materials are designed for optimum optical properties.

In an exemplary embodiment, a liquid crystalline material is homogenizedin the presence of finely divided silica, a coalescence limitingmaterial, (LUDOX® from duPont Corporation). A promoter material, such asa copolymer of adipic acid and 2-(methylamino)ethanol, is added to theaqueous bath to drive the colloidal particles to the liquid-liquidinterface. The liquid crystal material is dispersed using ultrasound tocreate liquid crystal domains below 1 micron in size. When theultrasound energy was removed, the liquid crystal material coalescedinto domains of uniform size. The ratio of smallest to largest domainsize varied by approximately 1:2. By varying the amount of silica andcopolymer relative to the liquid crystalline material, uniform domainsize emulsions of average diameter (by microscopy) approximately 1, 3,and, 8 micron were produced. These emulsions were diluted into gelatinsolution for subsequent coating.

Domains of a limited coalescent material maintain their uniform sizeafter the addition of the surfactant and after being machine coated.There were few, if any, parasitic domains having undesirableelectro-optical properties within the dried coatings produced by thelimited coalescence method. Coatings made using limited coalescencehaving a domain size of about 2 microns may have the greatesttranslucence. For constant material concentrations and coatingthickness, limited coalescent materials having a domain size of about 2microns in size are significantly more translucent than any sizeddomains formed using conventional dispersion.

Sheets made by the limited coalescence process have curves similar tothose of conventionally dispersed materials. However, with 8 to 10micron domains, the material may demonstrate reduced scattering due tothe elimination of parasitic domains. Conventionally dispersedcholesteric materials contain parasitic domains, which reflect light inwavelengths outside the wavelengths reflected by the cholestericmaterial. Limited coalescent dispersions have reduced reflection inother wavelengths due to the elimination of parasitic domains. Theincreased purity of color is important in the development of full colordisplays requiring well separated color channels to create a full colorimage. Limited coalescent cholesteric materials provide purer lightreflectance than cholesteric liquid crystal material dispersed byconventional methods. Such materials may be produced using conventionalphotographic coating machinery.

In order to provide suitable formulations for applying a layercontaining the liquid crystal domains, the dispersions are combined witha hydrophilic colloid, gelatin being the preferred material. Surfactantsmay be included with the liquid crystal dispersion prior to the additionof gelatin in order to prevent the removal of the particulate suspensionstabilizing agent from the droplets. This aids in preventing furthercoalescence of the droplets.

As for the suspension stabilizing agents that surround and serve toprevent the coalescence of the droplets, any suitable colloidalstabilizing agent known in the art of forming polymeric particles by theaddition reaction of ethylenically unsaturated monomers by the limitedcoalescence technique can be employed, such as, for example, inorganicmaterials such as, metal salt or hydroxides or oxides or clays, organicmaterials such as starches, sulfonated crosslinked organic homopolymersand resinous polymers as described, for example, in U.S. Pat. No.2,932,629; silica as described in U.S. Pat. No. 4,833,060; copolymerssuch as copoly(styrene-2-hydroxyethyl methacrylate-methacrylicacid-ethylene glycol dimethacrylate) as described in U.S. Pat. No.4,965,131, all of which are incorporated herein by reference. Silica isthe preferred suspension stabilizing agent.

Suitable promoters to drive the suspension stabilizing agent to theinterface of the droplets and the aqueous phase include sulfonatedpolystyrenes, alginates, carboxymethyl cellulose, tetramethyl ammoniumhydroxide or chloride, triethylphenyl ammonium hydroxide, triethylphenylammonium hydroxide, triethylphenyl ammonium chloride,diethylaminoethylmethacrylate, water soluble complex resinous aminecondensation products, such as the water soluble condensation product ofdiethanol amine and adipic acid, such as poly(adipicacid-co-methylaminoethanol), water soluble condensation products ofethylene oxide, urea, and formaldehyde and polyethyleneimine; gelatin,glue, casein, albumin, gluten, and methoxycellulose. The preferredpromoter is triethylphenyl ammonium chloride.

In order to prevent the hydrophilic colloid from removing the suspensionstabilizing agent from the surface of the droplets, suitable anionicsurfactants may be included in the mixing step to prepare the coatingcomposition such as polyisopropyl naphthalene-sodium sulfonate, sodiumdodecyl sulfate, sodium dodecyl benzene sulfonate, as well as thoseanionic surfactants set forth in U.S. Pat. No. 5,326,687 and in SectionXI of Research Disclosure 308119, December 1989, entitled “PhotographicSilver Halide Emulsions, Preparations, Addenda, Processing, andSystems”, both of which are incorporated herein by reference. Aromaticsulfonates are more preferred and polyisopropyl naphthalene sulfonate ismost preferred.

Suitable hydrophilic binders include both naturally occurring substancessuch as proteins, protein derivatives, cellulose derivatives, forexample, cellulose esters, gelatins and gelatin derivatives,polysaccaharides, casein, and the like, and synthetic water permeablecolloids such as poly(vinyl lactams), acrylamide polymers, poly(vinylalcohol) and its derivatives, hydrolyzed polyvinyl acetates, polymers ofalkyl and sulfoalkyl acrylates and methacrylates, polyamides, polyvinylpyridine, acrylic acid polymers, maleic anhydride copolymers,polyalkylene oxide, methacrylamide copolymers, polyvinyl oxazolidinones,maleic acid copolymers, vinyl amine copolymers, methacrylic acidcopolymers, acryloyloxyalkyl acrylate and methacrylates, vinyl imidazolecopolymers, vinyl sulfide copolymers, and homopolymer or copolymerscontaining styrene sulfonic acid. Gelatin is preferred.

Although hardened gelatin is used in photographs to harden the material,the need is not the same in liquid crystal displays in which the gelatinis typically protected by several layers of material including a plasticor glass substrate. Typically, liquid crystal material is wicked betweenplates of glass. Furthermore, unless necessary, a gelatin hardener canbe problematic for coating a gelatin material and may require moredifficult manufacture. However, gelatin containing hardener mayoptionally be used in the present invention. In the context of thisinvention, hardeners are defined as any additive which causes chemicalcrosslinking in gelatin or gelatin derivatives.

Many conventional hardeners are known to crosslink gelatin. Gelatincrosslinking agents, also referred to as the hardener, are included inan amount of at least about 0.01 wt. % and preferably from about 0.1 toabout 10 wt. % based on the weight of the solid dried gelatin materialused. By dried gelatin, it is meant that the gelatin is substantiallydry at ambient conditions as, for example, obtained from Eastman GelCo., as compared to swollen gelatin. More preferably, the gelatin ispresent in the amount of from about 1 to about 5 percent by weight. Morethan one gelatin crosslinking agent can be used if desired.

Inorganic hardeners include compounds such as aluminum salts, especiallythe sulfate, potassium and ammonium alums, ammonium zirconium carbonate,chromium salts such as chromium sulfate and chromium alum, and salts oftitanium dioxide, zirconium dioxide, and the like. Representativeorganic hardeners or gelatin crosslinking agents useful in the presentinvention may include aldehyde and related compounds, pyridiniums,olefins, carbodiimides, and epoxides. Thus, suitable aldehyde hardenersinclude formaldehyde and compounds that contain two or more aldehydefunctional groups such as glyoxal, gluteraldehyde and the like. Otherpreferred hardeners include compounds that contain blocked aldehydefunctional groups such as aldehydes of the typetetrahydro-4-hydroxy-5-methyl-2(1H)-pyrimidinone polymers (Sequa SUNREZâ700), polymers of the type having a glyoxal polyol reaction productconsisting of 1 anhydroglucose unit: 2 glyoxal units (SEQUAREZâ 755obtained from Sequa Chemicals, Inc.), DME-Melamine non-formaldehyderesins such as Sequa CPD3046-76 obtained from Sequa Chemicals Inc.,2,3-dihydroxy-1,4-dioxane (DHD), and the like. Thus, hardeners thatcontain active olefinic functional groups include, for example,bis-(vinylsulfonyl)-methane (BVSM), bis-(vinylsulfonyl-methyl) ether(BVSME), 1,3,5-triacryloylhexahydro-s-triazine, and the like. In thecontext of the present invention, active olefinic compounds are definedas compounds having two or more olefinic bonds, especially unsubstitutedvinyl groups, activated by adjacent electron withdrawing groups (TheTheory of the Photographic Process, 4th Edition, T. H. James, 1977,Macmillan Publishing Co., page 82). These compounds can be readilyprepared using the published synthetic procedure or routinemodifications that would be readily apparent to one skilled in the artof synthetic organic chemistry.

Other examples of hardening agents can be found in standard referencessuch as The Theory of the Photographic Process, T. H. James, MacmillanPublishing Co., Inc. (New York 1977) or in Research Disclosure,September 1996, Vol. 389, Part IIB (Hardeners) or in ResearchDisclosure, September 1994, Vol. 365, Item 36544, Part IIB (Hardeners).Research Disclosure is published by Kenneth Mason Publications, Ltd.,Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England. Asindicated above, both inorganic and organic hardeners are known and canbe used in the present invention. Organic hardening agents are preferredover inorganic hardeners.

Olefinic hardeners are most preferred. As mentioned above, olefinichardeners are compounds with active olefinic functionality, includingdivinyl ketone, resorcinol bis(vinylsulfonate) (U.S. Pat. No. 3,689,274,incorporated herein in by reference), 4,6-bis(vinylsulfonyl)-m-xylene(U.S. Pat. No. 2,994,611, incorporated herein in by reference),bis(vinylsulfonylalkyl) ethers and amines (U.S. Pat. No. 3,642,486 andU.S. Pat. No. 3,490,911, incorporated herein in by reference),1,3,5-tris(vinylsulfonyl) hexahydro-s-triazine, diacrylamide (U.S. Pat.No. 3,635,718, incorporated herein in by reference),1,3-bis(acryloyl)urea (U.S. Pat. No. 3,640,720, incorporated herein inby reference), N,N′-bismaleimides (U.S. Pat. No. 2,992,109, incorporatedherein in by reference) bisisomaleimides (U.S. Pat. No. 3,232,763,incorporated herein in by reference), bis(2-acetoxyethyl) ketone (U.S.Pat. No. 3,360,372, incorporated herein in by reference), and1,3,5-triacryloylhexahydro-s-triazine. Blocked active olefins of thetype bis(2-acetoxyethyl) ketone and 3,8-dioxodecane-1,10-bis(pyridiniumperchlorate) may also be used.

Among hardeners of the active olefin type, a preferred class ofhardeners particularly are compounds comprising two or more vinylsulfonyl groups. These compounds are hereinafter referred to as “vinylsulfones.” Compounds of this type are described in numerous patentsincluding, for example, U.S. Pat. Nos. 3,490,911, 3,642,486, 3,841,872and 4,171,976, incorporated herein in by reference. Vinyl sulfonehardeners are believed to be effective as hardeners as a result of theirability to crosslink polymers making up the colloid.

A preferred class of vinyl sulfone hardeners for use in this inventionare compounds of the formula:(H₂C═CH—SO₂)n-Zwherein n is an integer with a value of 2 to 6 and Z is an organiclinking group with a valence equal to n. Suitable examples of theorganic linking group represented by Z include alkyl, alkylene, aryl,arylene, aralkyl and alkaryl groups. As a further example Z can be aheteroatom such as a nitrogen atom or an ether oxygen atom.

In the above formula Z is preferably—A——O—A—O—, or—D—where A is an alkylene group containing 1 to 8 carbon atoms which may beunsubstituted or substituted and the alkylene chain may be interruptedby one or more hetero atoms or organic groups, or an arylene group,which may be substituted or unsubstituted, and D is a trivalent alkylenegroup, a trivalent arylene group which may be substituted with one ormore additional CH₂═CH—SO₂— groups, a trivalent cyclic alkylene groupwhich may be substituted with one or more CH₂═CH—SO₂— groups, or atrivalent heterocyclic group which may be substituted with one or moreCH₂═CH—SO₂— groups. Preferred substituents for A include —OH, phenyl,aralkyl, such as phenethyl, or CH₂═CH—SO₂— groups. The aryl moiety ofthe aralkyl group may be sulfonated. The alkylene group may beinterrupted by one or more of the following: oxygen atoms, arylenegroups, cycloalkyl groups, —NHCONH—, or —N—R, where R is an alkyl groupcontaining 1 to 8 carbon atoms.

A particularly preferred class of vinyl sulfone hardeners for use inthis invention are bis(vinylsulfonyl)alkane hardeners of the formula:CH₂═CH—SO₂—(CH₂)x-SO₂—CH═CH₂where x is an integer with a value of from 1 to 3.

A preferred vinyl sulfone hardener for use in this invention isbis(vinylsulfonyl)methane (BVSM) which has the formula:CH₂═CH—SO₂—CH₂—SO₂—CH═CH₂

Another preferred vinyl sulfone hardener for use in this invention isbis(vinylsulfonylmethyl)ether (BVSME) which has the formula:CH₂═CH—SO₂—CH₂—O—CH₂—SO₂—CH═CH₂

The vinyl sulfone hardeners described herein can be used in anyeffective amount in hardening gelatin. Suitable amounts are typically inthe range of from about 0.5 to about 10 percent by weight, based on theweight of hydrophilic colloid, and more preferably in the amount of fromabout 1 to about 5 percent by weight.

The LCD may contain at least one transparent conductive layer, whichtypically is comprised of a metal or preferably a metal oxide. Theconductive metals can include any metal but preferably a highconductivity metal such as gold, silver, platinum, copper, aluminum,indium, tin, palladium, vanadium, chromium, iron, cobalt, nickel and ormixtures thereof. The conductive metal oxides can include indium oxide,titanium oxide, cadmium oxide, gallium indium oxide, niobium oxide, tinoxide, indium tin oxide and the like. See, Int. Publ. No. WO 99/36261 byPolaroid Corporation, incorporated herein in by reference. In additionto the primary oxide such as indium tin oxide (ITO), the at least oneconductive layer can also comprise a secondary metal oxide such as anoxide of cerium, titanium, zirconium, hafnium and/or tantalum. See, U.S.Pat. No. 5,667,853 to Fukuyoshi et al, incorporated herein in byreference. Other transparent conductive oxides include, but are notlimited to ZnO₂, Zn₂SnO₄, Cd₂SnO₄, Zn₂In₂O₅, Mgln₂O₄, Ga₂O₃—In₂O₃, orTaO₃. The conductive layer may be formed, for example, by a lowtemperature sputtering technique or by a direct current sputteringtechnique, such as DC-sputtering or RF-DC sputtering, depending upon thematerial or materials of the underlying layer.

Alternative to metals or metal oxides, electronically conductivepolymers can also be used for the transparent conductive layer. In thisregard, any of the conductive polymers discussed herein above for use inthe color contrast conductive layer can be used. Particularly suitableelectronically conductive polymer layers are those comprisingpolythiophene with a “figure of merit (FOM)”, as described in U.S. Ser.Nos. 10/944,570 and 10/969,889, incorporated herein in by reference, of<150, preferably <100, and more preferably <50.

Another type of conductive material that can be used in the transparentconductive layer includes carbon nanotubes such as single walled ormultiwalled carbon nanotubes.

Indium tin oxide (ITO) is the preferred transparent conductive material,because of its widespread availability and desired optical andelectrical properties, such as up to 90% transmission, and down to 20ohms per square surface resistivity. An exemplary preferred ITO layerhas a % T greater than or equal to 80% in the visible region of light,that is, from greater than 400 nm to 700 nm, so that the film will beuseful for display applications. In a preferred embodiment, theconductive layer comprises a layer of low temperature ITO which ispolycrystalline. The ITO layer is preferably 10-120 nm in thickness, or50-100 nm thick to achieve a resistivity of <1000 ohms/square onplastic.

The conductive layer is preferably patterned. The conductive layer ispreferably patterned into a plurality of electrodes. The patternedelectrodes may be used to form a LCD device. In another embodiment, twoconductive substrates are positioned facing each other and cholestericliquid crystals are positioned therebetween to form a device. In thecase of a bistable display, some mechanism is provided to produce afield, which acts upon the electrically modulated display to produce achange of state in the modulated layer. The patterned ITO conductivelayer may have a variety of dimensions. Exemplary dimensions may includeline widths of 10 microns, distances between lines, that is, electrodewidths, of 200 microns, depth of cut, that is, thickness of ITOconductor, of 100 nanometers. ITO thicknesses on the order of 60, 70,and greater than 100 nanometers are also possible.

The LCD may also comprise at least one “functional layer.” Thefunctional layer may include any number layers such as antistaticlayers, tie layers or adhesion promoting layers, abrasion resistantlayers, curl control layers, conveyance layers, protective or barrierlayers, dielectric layers, splice providing layers, UV absorptionlayers, optical effect providing layers, such as antireflective andantiglare layers, waterproofing layers, adhesive layers, imaging layersand the like. Preferred functional layers include protective or barrierlayers, antistatic layers, dielectric layers and adhesive layers. It isalso possible to combine more than one functional layer in to a singlelayer with multiple attributes. For example, a functional layer may actas a dielectric layer and an adhesive layer; similarly, a functionallayer may act as a barrier layer and a dielectric layer. The functionallayer of the invention can be applied in any of a number of well knowntechniques, such as dip coating, rod coating, blade coating, air knifecoating, gravure coating and reverse roll coating, extrusion coating,slide coating, curtain coating, and the like.

A preferred barrier layer may acts as a gas barrier or a moisturebarrier and may comprise SiOx, AlOx or ITO. The protective layer, forexample, can include an acrylic hard coat, that functions to preventlaser light from penetrating to functional layers between the protectivelayer and the substrate.

In another embodiment, the polymeric support may further comprise anantistatic layer to manage unwanted charge build up on the sheet or webduring roll conveyance or sheet finishing. In another embodiment of thisinvention, the antistatic layer has a surface resistivity of between 10⁵to 10¹² ohms/square. Above 10¹², the antistatic layer typically does notprovide sufficient conduction of charge to prevent charge accumulationto the point of preventing fog in photographic systems or from unwantedpoint switching in liquid crystal displays. While layers greater than10⁵ will prevent charge buildup, most common antistatic materials areinherently not that conductive and in those materials that are moreconductive than 10⁵, there is usually some color associated with themthat can reduce the overall transmission properties of the display. Theantistatic layer is separate from the highly conductive electrodelayers, i.e., the color contrast layer and the transparent conductivelayer and provides the best static control when it is on the oppositeside of the web substrate from that of the electrode layers. This mayinclude the web substrate itself.

The functional layer may also comprise a dielectric material. Adielectric layer, for purposes of the present invention, is a layer thatis not conductive or blocks the flow of electricity. This dielectricmaterial may include a UV curable, thermoplastic, screen printablematerial, such as Electrodag 25208 dielectric coating from AchesonCorporation. The dielectric material forms a dielectric layer. Thislayer may include openings to define image areas, which are coincidentwith the openings. Since the image is viewed through a transparentsubstrate, the indicia are mirror imaged.

The dielectric material may form an adhesive layer to subsequently bondan electrode to the light modulating layer. Conventional laminationtechniques involving heat and pressure are employed to achieve apermanent durable bond. Certain thermoplastic polyesters, such as VITEL1200 and 3200 resins from Bostik Corp., polyurethanes, such as MORTHANECA-100 from Morton International, polyamides, such as UNIREZ 2215 fromUnion Camp Corp., polyvinyl butyral, such as BUTVAR B-76 from Monsanto,and poly(butyl methacrylate), such as ELVACITE 2044 from ICI AcrylicsInc. may also provide a substantial bond between the electricallyconductive and light modulating layers.

The dielectric adhesive layer may be coated from common organic solventsat a dry thickness of one to three microns. The dielectric adhesivelayer may also be coated from an aqueous solution or dispersion.Polyvinyl alcohol, such as AIRVOL 425 or MM-51 from Air Products,poly(acrylic acid), and poly(methyl vinyl ether/maleic anhydride), suchas GANTREZ AN-119 from GAF Corp. can be dissolved in water, subsequentlycoated over the second electrode, dried to a thickness of one to threemicrons and laminated to the light modulating layer. Aqueous dispersionsof certain polyamides, such as MICROMID 142LTL from Arizona Chemical,polyesters, such as AQ 29D from Eastman Chemical Products Inc.,styrene/butadiene copolymers, such as TYLAC 68219-00 from ReichholdChemicals, and acrylic/styrene copolymers such as RayTech 49 and RayKote234L from Specialty Polymers Inc. can also be utilized as a dielectricadhesive layer as previously described.

Layers in the various embodiments may include radiation curable layers.The curing process can occur by any means known to those of skill in theart of curing coatings, such as through application of light, heat,airflow, chemical reaction, or some other source of energy. Lightactivation of the curing process can occur through exposure toultraviolet, visible, infrared light, or combinations thereof, whichthen initiates a chemical reaction to cure the materials, such asthrough crosslinking polymerizations.

The following examples are provided to illustrate the invention.

Example 1

The following ingredients were used to form the coating composition forthe various color contrast conductive layers:

Ingredients for Coating Composition

(a) Baytron P HC: aqueous dispersion of electronically conductivepolythiophene and polyanion, namely, poly(3,4-ethylene dioxythiophenestyrene sulfonate), supplied by H.C. Starck;

(b) Olin 10G: nonionic surfactant supplied by Olin Chemicals;

(c) N-methylpyrrolidone: conductivity enhancing agent supplied by Acros;

(d) diethylene glycol: conductivity enhancing agent supplied by Aldrich;

(e) Silquest A 187: 3-glycidoxy-propyltrimethyoxysilane supplied byCrompton Corporation and

(f) isopropanol;

(g) carbon dispersion prepared as follows:

A mixture of 325 g of polymeric beads having mean diameter of 50 μm,62.5 g of Pigment Black 7 (Cabot Corp.); 15.62 g of potassium oleoylmethyl taurate (KOMT); and 187.5 g of deionized water was prepared.These components were milled for 8 hours in a double-walled vessel atroom temperature using a high-energy media mill manufactured byMorehouse-Cowles Hochmeyer.

The mixture was filtered through a 4-8 μm Buchner funnel to remove thepolymeric beads, and the resulting filtrate diluted to give a carbondispersion having a 12 wt. % final concentration of pigment. Kordek 50 Cbiocide (Rohm and Haas) was added to the dispersion at an amountnecessary to give a final concentration of 500 ppm. The median particlesize of the pigment dispersion was 70 nm, as determined using aMICROTRAC II Ultrafine particle analyzer (UPA) manufactured by Leeds &Northrup.

The following coating compositions A, B, C and D were prepared forcoating suitable substrates to form the color contrast conductive layerexamples:

Coating composition A (invention) Baytron P HC (1.3% aqueous) 70.97 gCarbon dispersion (1.3% aqueous) 17.74 g Olin 10G (10% aqueous) 0.5 gN-methylpyrrolidone 5.16 g Diethylene glycol 4 g Silquest A 187 1.8 gIsopropanol 4.33 g

Coating composition B (invention) Baytron P HC (1.3% aqueous) 62.10 gCarbon dispersion (1.3% aqueous) 26.61 g Olin 10G (10% aqueous) 0.5 gN-methylpyrrolidone 5.16 g Diethylene glycol 4 g Silquest A 187 1.8 gIsopropanol 4.33 g

Coating composition C (invention) Baytron P HC (1.3% aqueous) 44.36 gCarbon dispersion (1.3% aqueous) 44.35 g Olin 10G (10% aqueous) 0.5 gN-methylpyrrolidone 5.16 g Diethylene glycol 4 g Silquest A 187 1.8 gIsopropanol 4.33 g

Coating composition D (comparative) Baytron P HC (1.3% aqueous) 88.71 gCarbon dispersion (1.3% aqueous) 0 g Olin 10G (10% aqueous) 0.5 gN-methylpyrrolidone 5.16 g Diethylene glycol 4 g Silquest A 187 1.8 gIsopropanol 4.33 g

The coating compositions were hopper coated on photographic grade 102 μmPET substrate subbed with an adhesion promoting subbing layer (on thesubbing side). The said subbing layer comprised a vinylidenechloride-acrylonitrile-acrylic acid terpolymer latex. The wet lay downof the coating composition was varied to obtain different surfaceelectrical resistivity (SER) and visual light transmission (% T)combinations, upon drying. Drying was carried out at ˜82 C for 5minutes. The details of the coatings, Examples 1 through 6 as perinvention and comparative sample Comp. 1, are listed in Table 1 below.

Baytron P HC/carbon Coating pigment SER Sample composition weight ratioohms/square % T Example 1 A 80/20 593 18 Example 2 B 70/30 704 12.6Example 3 C 50/50 1382 4.5 Example 4 A 80/20 300 3.37 Example 5 B 70/30400 2.61 Example 6 C 50/50 1000 1.4 Comp. 1 D 100/0  298 70 Comp. 2 D 0/100 uncoatable —It is very clear that various ranges of SER, from 1000 to 300ohm/square, and % T, from 18 to 1.4%, can be obtained for Examples 1-6,as per the invention, to generate desirable color contrast conductivelayers, with low SER and low % T. In contrast, comparative sample Comp.1illustrates that without the carbon pigment, the SER can be as low as298 ohm/square but with % T at an unacceptably high value of 70%.

Thus, the present invention overcomes a significant limitation of thestate of the art, wherein two separate layers are coated forelectrically imageable display, one for color contrast and the other forconductivity, by combining them into an aqueously coatable single layerwith low SER and low % T.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. A bistable reflective display comprising a substrate, an electricallymodulated imaging layer, and a coatable color contrast conductive layercomprising an electronically conductive polymer and colorant, whereinthe ratio of said polymer to said colorant is in the range from about95:5 to about 40:60.
 2. The bistable reflective display of claim 1wherein said substrate is flexible.
 3. The bistable reflective displayof claim 1 wherein said substrate comprises polyester.
 4. The bistablereflective display of claim 1 wherein said substrate comprises cellulosetriacetate, also known as triacetylcellulose or TAC.
 5. The bistablereflective display of claim 1 wherein said electrically modulatedimaging layer comprises a light modulating material.
 6. The bistablereflective display of claim 5 wherein said light modulating material isa chiral nematic liquid crystal material.
 7. The bistable reflectivedisplay of claim 5 wherein said electrically modulated imaging layercomprises a polymer dispersed cholesteric liquid crystal layer.
 8. Thebistable reflective display of claim 7 wherein said polymer within whichsaid cholesteric liquid crystal is dispersed is gelatin.
 9. The bistablereflective display of claim 7 wherein said polymer within which saidcholesteric liquid crystal is dispersed is water soluble.
 10. Thebistable reflective display of claim 1 wherein said electronicallyconductive polymer is polythiophene.
 11. The bistable reflective displayof claim 10 wherein said polythiophene is present in a cationic formwith a polyanion.
 12. The bistable reflective display of claim 1 whereinsaid color contrast conductive layer has a surface electricalresistivity of less than 10⁴ ohms/sq.
 13. The bistable reflectivedisplay of claim 1 wherein said color contrast conductive layer has athickness less than 1.0 micron.
 14. The bistable reflective display ofclaim 1 wherein the thickness of said color contrast conductive layer isless than 25% of the said electrically modulated imaging layer.
 15. Thebistable reflective display of claim 1 wherein said color contrastconductive layer has visual light transmission of ≦10%.
 16. The bistablereflective display of claim 1 wherein said colorant comprises carbon.17. The bistable reflective display of claim 1 wherein said colorcontrast conductive layer provides a background that is substantiallyneutral to the human eye.
 18. The bistable reflective display of claim 1wherein said color contrast conductive layer provides a background thatprovides a substantially neutral optical density such that there isvariability of less than +/−20% from the mean optical density over atleast 80% of the visible spectrum from 400 to 700 nm.
 19. The bistablereflective display of claim 1 wherein said color contrast conductivelayer comprises a combination of at least two nonconductive colorantswhich have different hues.
 20. The bistable reflective display of claim1 wherein said color contrast conductive layer comprises cyan, magenta,and yellow nonconductive colorants.
 21. The bistable reflective displayof claim 20 wherein said colorant comprises a pigment particle having amedian particle diameter of less than 50 percent of the thickness ofsaid color contrast conductive layer.
 22. The bistable reflectivedisplay of claim 1 wherein said color contrast conductive layer furthercomprises a conductivity enhancing agent (CEA).
 23. The bistablereflective display of claim 1 further comprising a transparentelectrically conductive layer, wherein said electrically modulatedimaging layer is between said color contrast conductive layer and saidtransparent electrically conductive layer.
 24. A method for making abistable reflective display comprising providing a substrate, applying atransparent conductive layer, applying an electrically modulated imaginglayer to said substrate and said transparent conductive layer, andadding a color contrast conductive layer comprising an electronicallyconductive polymer and colorant, wherein the ratio of the electricallyconductive polymer to the colorant is in the range from about 95:5 toabout 40:60.
 25. A bistable reflective display comprising a substrate,an electrically modulated imaging layer, and a coatable color contrastconductive layer comprising an electronically conductive polymer andcolorant, wherein said color contrast conductive layer has visual lighttransmission of ≦10%.
 26. The bistable reflective display of claim 25,wherein said color contrast conductive layer provides a background thatprovides a substantially neutral optical density such that there isvariability of less than +/−20% from the mean optical density over atleast 80% of the visible spectrum from 400 to 700 nm.
 27. A bistablereflective display comprising a substrate, an electrically modulatedimaging layer, and a coatable color contrast conductive layer comprisingan electronically conductive polymer and colorant, wherein said colorcontrast conductive layer provides a background that provides asubstantially neutral optical density such that there is variability ofless than +/−20% from the mean optical density over at least 80% of thevisible spectrum from 400 to 700 nm.